This invention relates to a high-pressure reciprocating pump having a cylinder, a plunger slideably guided therein, and at least one inlet/outlet port for the pumped medium, said port being intermittently blocked by the plunger, where the cylinder wall is in its drive-end area shaped to form a guide surface for the plunger and where an annular recess is provided in the plunger chamber end area, which is spaced apart from the inlet/outlet port by a first cylindrical guide surface.
A high-pressure reciprocating pump of this description has been disclosed in German patent specification 1653500 and is used as an injection pump for engines, more particularly diesel engines.
In advanced designs, the injection pressure is increasingly raised; it has reached about 1500 bar. At these elevated pressures the plunger, although given very little running clearance, may easily cant during the high-pressure phase under high lateral forces and rub against the guide. This may cause the plunger to seize on the cylinder wall.
Endeavors have previously been made to prevent such seizing. For example, German patent specification 2741348 describes a plunger into which several circumferential slots communicating with the filling chamber are recessed.
British Patent Documents GB 724 986 and GB 2 077 862 disclose high-pressure reciprocating pumps of this generic description having means for preventing the exit of pumped medium along the drive direction between the plunger and the cylinder, where the annular recess is provided to facilitate manufacture.
In a broad aspect, the present invention provides a high-pressure reciprocating pump of the above generic description for use at high pressure without the risk of seizing.
It is a particular object of the present invention to provide a reciprocating pump where on the drive side with reference to the inlet/outlet port, a second annular 0.2 to 1.0 times the plunger diameter, said recess being separated from the inlet/outlet port by a second guide surface, where the first and the second guide surfaces have an axial chord of 0.05 to 0.5 d, and where both annular recesses have a depth of 0.1 to 3 mm referred to the guide surfaces.
The present invention provides an essential advantage in that with increasing plunger travel and pump pressure, the plunger emerges form the guide surface and so reduces its surface areas exposed to the application of unilateral side forces, because the uniform increase of the emerging circumferential surface of the plunger causes the latter to be pressurized uniformly all around. This causes the unilateral side forces induced by canting to be appreciably alleviated by the newly created, circumferentially balanced side forces. The farther the travel of the plunger towards top dead center causes the plunger chamber pressure to rise, the farther the plunger emerges into the recessed area behind the guide surface to be uniformly laterally pressurized on this growing circumferential plunger surface.
If the axial chord of the guide surface (e.sub.1) is made too large, more particularly larger than 0.5.times.d, as it is the case on conventional high-pressure reciprocating pumps, the relative growth in balanced side forces is not sufficiently large to prevent rubbing or seizing also under peak pressures. If, on the other hand, e.sub.1 is made too small, more particularly smaller than 0.05.times.d, adequate sealing is no longer ensured.
A range of 0.1 to 0.3.times.d has shown to be an especially beneficial value of e.sub.1. The exact value of e.sub.1 to select will depend on the prevailing operating conditions, especially the pressures.
The annular recess on the drive side causes further surface areas to be relieved of lateral pressure, so that an important portion of the plunger surface is balanced circumferentially that might otherwise apply undesirable lateral pressure on the plunger exactly in the starting phase of pressurization. From this aspect, an axial chord of the annular slot of n=0.2 to 2.0.times.d has proved especially beneficial. Normally the distance e.sub.2 is made approximately equal to e.sub.1. Depending on pump design operating conditions and pressures, however, other values may be preferable.
In a further advantageous aspect of the present invention the plunger, as previously disclosed, has a circumferential slot communicating with the plunger face, where from said circumferential slot in the drive direction the plunger has a conical shape over a limited section (k). The pressure of the pumped medium incident into the conical gap advantageously initiates the parallel position of the plunger relative to the cylinder centerline. It additionally reduces the side forces acting on the plunger.
This arrangement is especially advantageous in conjunction with the annular slot described above, because when combined, these two features facilitate the centering of the plunger during the initial phase of pressure build-up. The length (k) of this conical section should be selected such that when the position of the plunger in the guide is that prevailing at the beginning of delivery, no communication exists between the plunger chamber and the annular slot. The angle alpha with the plunger centerline amounts to 0.2' to 10' (minutes).